Thin film photovoltaic (PV) modules (also referred to as “solar panels” or “solar modules”) are gaining wide acceptance and interest in the industry as a viable source of alternate energy, particularly modules based on cadmium telluride (CdTe) paired with cadmium sulfide (CdS) as the photo-reactive components. Solar energy systems using CdTe PV modules are generally recognized as the most cost efficient of the commercially available systems in terms of cost per watt of power generated. However, the advantages of CdTe not withstanding, sustainable commercial exploitation and acceptance of solar power as a supplemental or primary source of industrial or residential power depends on the ability to produce efficient PV modules on a large scale and in a cost effective manner. In this regard, the high cost of the photo-reactive source materials used in the production of conventional solar panels is a limiting factor, and maximizing the utilization of these materials is of primary importance.
In a continuous vapor deposition process, the individual glass sheets (substrates) are continuously conveyed through a deposition apparatus, as compared to a batch process wherein the substrates are indexed into and out of the deposition apparatus. The ingress (entrance) and egress (exit) seals through which the substrates enter and leave the deposition apparatus are, however, an inherent leakage source for the sublimated source material. Slit seals are typically provided having the smallest slit reasonably possible through which the substrates travel. However, variations in substrate thickness, size, flatness, and other dimensional anomalies, often results in the substrates rubbing against the slit seal members, resulting in scratches and other flaws in the film layer or even jamming of the substrates.
U.S. Pat. No. 5,772,715 describes a deposition system wherein multiple vacuum chamber slit seals are provided at the ingress and egress locations. These seals are relatively complicated and involve establishing three separate vacuum stages drawn by separate vacuum pumps. Although the U.S. '715 patent describes that the seals are suitable for processing either discrete glass sheets or a continuous glass ribbon, they are mechanically quite complex and would add significantly to the manufacturing costs of PV modules.
U.S. Pat. No. 6,058,740 describes a deposition system with upper and lower seal rolls provided at the ingress and egress seal locations. The rolls are driven and engage and roll against the substrates as they enter and exit the vacuum deposition chamber. This system, however, relies on frictional rolling engagement of the seal rolls against the surface of the thin film layer at the egress point, which may contribute to scratching, marring, or other induced flaws in the film layer.
Accordingly, there exists an ongoing need in the industry for a seal configuration that is particularly suited for large scale and efficient production of PV modules, particularly CdTe based modules, by continuous conveyance of a plurality of discrete substrates through a vapor deposition chamber.